FIG. 28(a) is a block circuit diagram of a first conventional switching DC-DC converter of step-down type. As shown in the figure, a smoothing capacitor Cin for smoothing a DC input voltage Vin, a main switching transistor 1 as a main switching semiconductor device for connecting and disconnecting the DC input voltage Vin, a freewheel diode 11, a coil L, a capacitor Cout, a control circuit 3, a gate driver (hereinafter referred to simply as a "driver") 2, and passive components 4 are provided. In FIG. 28(a), the main switching transistor 1 is a p-channel MOSFET.
The freewheel diode 11 provides a load current path when the main switching transistor 1 is OFF. The coil L and the capacitor Cout are disposed to smooth an output voltage Vout from the DC-DC converter.
The input voltage Vin is connected, as a power supply, to the control circuit 3 and the driver 2. The control circuit 3 detects the output voltage Vout and outputs a control signal 3a for setting the timing of switching on and off the main switching transistor 1 to keep the output voltage Vout at a certain value. The driver 2 outputs a drive signal 2a in response to the control signal 3a from the control circuit 3 to directly switch on and off the main switching transistor 1.
Various attached passive components for the control circuit 3 are collectively represented by the passive components 4 in FIG. 28(a). For example, a capacitor for smoothing the output of the constant voltage source in the control circuit 3, a resistor and a capacitor for setting the oscillator frequency in the control circuit 3 are included in the passive components 4.
FIG. 28(b) is a block circuit diagram of a second conventional switching DC-DC converter of step-down type. FIG. 28(b) shows only a part of the second conventional DC-DC converter different from the corresponding part of the first conventional DC-DC converter. The second conventional DC-DC converter is different from the first conventional DC-DC converter in that the freewheel diode 11 of FIG. 28(a) is replaced by a synchronous commutation transistor 12 formed of an N-channel MOSFET in FIG. 28(b).
The synchronous commutation transistor 12 is switched off and on in response to the drive signal 2b from the driver 2 synchronizing with the ON and OFF of the main switching transistor 1 to provide a load current path when the main switching transistor 1 is OFF.
Since the on-voltage of the transistor may be set lower than the forward voltage drop across the diode, the synchronous commutation transistor 12 used in substitution for the freewheel diode 11 facilitates improving the efficiency of the switching DC-DC converter.
FIG. 29 is a block circuit diagram showing a circuit configuration that integrates the main switching transistor 1, the driver 2 and the control circuit 3 of FIG. 28(a) in a one-chip semiconductor integrated circuit (hereinafter referred to as a "one-chip IC") 100.
In the circuit configuration shown in FIG. 29, the operation of the converter becomes unstable when the load is heavy, presumably because the control circuit malfunctions due to an increase of the switching noise caused by load current increase, and it becomes impossible for the converter to operate properly as the switching frequency increases.
The conditions are worse for the DC-DC converter shown in FIG. 28(b) that incorporates the synchronous commutation transistor 12 in the one-chip IC 100, since the switching noise from the synchronous commutation transistor 12 is added.
In view of the foregoing, it is an object of the invention to provide a DC-DC converter, including a main switching transistor, a driver and a control circuit incorporated in a one-chip IC, that facilitates preventing switching noises, caused by a heavily load or by high-speed switching, from affecting the control circuit adversely and securing stable operations of the control circuit.